Which statement best describes the flow characteristics of positive-displacement pumps compared with centrifugal pumps in marine propulsion systems?

Which statement best describes the flow characteristics of positive-displacement pumps compared with centrifugal pumps in marine propulsion systems?

• A. Positive-displacement pumps deliver a fixed volume per rotation at a given speed; centrifugal pumps provide flow proportional to speed and head dependent on the system.
• B. Positive-displacement pumps deliver flow proportional to speed; centrifugal pumps deliver fixed volume per rotation.
• C. Both pump types deliver identical flow regardless of system head.
• D. Centrifugal pumps deliver a fixed flow regardless of speed; positive-displacement pumps are independent of system head.

Answer: (A)

The key idea is how flow responds to speed and system resistance. A positive-displacement pump moves a fixed amount of fluid with each revolution, so at a given rotational speed the flow rate is essentially fixed (volume per rotation times speed). The pressure it can generate rises with back pressure, but the amount of fluid it delivers per unit time doesn’t change much over a range of heads. A centrifugal pump, on the other hand, relies on an impeller to impart energy to the fluid, so the flow rate changes with speed—flow increases roughly proportional to speed—and the actual flow is set by the intersection of the pump’s performance curve with the system’s head (piping, valves, and load). In marine propulsion terms, the system head limits centrifugal flow, while a PD pump keeps a constant flow at a given speed regardless of head (within practical limits). Therefore, the statement correctly describes that positive-displacement pumps deliver a fixed volume per rotation at a given speed, while centrifugal pumps deliver flow proportional to speed with head determined by the system.

The key idea is how flow responds to speed and system resistance. A positive-displacement pump moves a fixed amount of fluid with each revolution, so at a given rotational speed the flow rate is essentially fixed (volume per rotation times speed). The pressure it can generate rises with back pressure, but the amount of fluid it delivers per unit time doesn’t change much over a range of heads. A centrifugal pump, on the other hand, relies on an impeller to impart energy to the fluid, so the flow rate changes with speed—flow increases roughly proportional to speed—and the actual flow is set by the intersection of the pump’s performance curve with the system’s head (piping, valves, and load). In marine propulsion terms, the system head limits centrifugal flow, while a PD pump keeps a constant flow at a given speed regardless of head (within practical limits). Therefore, the statement correctly describes that positive-displacement pumps deliver a fixed volume per rotation at a given speed, while centrifugal pumps deliver flow proportional to speed with head determined by the system.